1. Technical Field
This invention relates generally to a system and method for suppressing impulse noise in an ion current processing system of the type useful in connection with controlling ignition in an internal combustion engine.
2. Description of the Related Art
One approach for detecting a combustion condition, such as knock or misfire, involves the use of a so-called ion sense system. It is known that the combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. It is further known to apply a relatively high voltage across, for example, the electrodes of a spark plug just after ignition in order to produce a current between the electrodes. Such current is known as an ion current. The ion current that flows is, generally speaking, proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above. Additionally, the level of such ion current may provide some measure of the level of ionization throughout the entire cylinder as combustion occurs. The DC level or amount of ion current is indicative of the quality of the combustion event, or whether in fact combustion has occurred at all (e.g., a misfire condition). An AC component of the ion current may be processed to determine the presence of knock. The ion sense approach is effective for any number of cylinder engines and various engine speed and load combinations.
For example, U.S. Pat. No. 5,534,781 issued to Lee et al. entitled xe2x80x9cCOMBUSTION DETECTION VIA IONIZATION CURRENT SENSING FOR A xe2x80x98COIL-ON-PLUGxe2x80x99 IGNITION SYSTEMxe2x80x9d discloses an ion sense system of the type described above having a ion sense voltage source and an integrator wherein the integrator develops an analog output that is an integrated version of an ion current signal. This analog output is provided to an electronic control unit. In ignition systems of the type disclosed in Lee et al. that utilize the spark plug electrodes for ionization sensing, a problem arises in that impulse noise spikes may be impressed upon the ion current signal as a result of corona partial discharges from the high voltage dielectric materials contained in the ignition system components. For example, insulators in the ignition coil, spark plugs, and spark plug wires. Ignition systems that utilize ion current sensing in order to control combustion xe2x80x9cknockxe2x80x9d often depend on an integration function, such as described in Lee et al., for processing the knock signal. These systems are adversely affected by the presence of the corona discharge voltage spikes (i.e., impulse noise), which may advance the integrator incrementally upwards, even in the absence of knock, and thereby cause false retard of the spark timing.
One known approach taken in the art for dealing with the above-identified problem involves observing the ion current signal within the integrator window for a high amplitude, relatively short duration burst, which is characteristic of the above-described corona partial discharge phenomenon. When such a signature characteristic is detected, such known system is configured to simply discard or ignore the knock signal for that particular combustion event. Such known approach, however, can be improved upon in as much as in the known system, no data concerning knock is used at all, even though for the most part (other than the abnormal contribution to the knock signal due to the impulse spike), the data can prove useful.
There is therefore a need to provide an improved apparatus for detecting a combustion condition, such as knock, that minimizes or eliminates one or more of the shortcomings as set forth above.
One object of the present invention is to provide a solution to one or more of the above-identified problems. One advantage of the present invention is that it allows the ability to provide a close-loop knock control system, while eliminating the need for discrete knock sensors and associated harnesses used in conventional systems. Another advantage of the present invention is that it provides an apparatus that provides knock data that will be valid for more, if not all, combustion events, unlike the conventional approach set forth in the Background which discards knock data when impulse noise is detected. A still further advantage of the present invention, in one embodiment, is that it allows mechanization for non-digital processing of a knock intensity signal, which may be located at the ignition coil, and thereafter communicated to an engine control module or the like.
An apparatus according to the present invention includes an ion current detection circuit and a processing circuit. The ion current detection circuit is configured to bias a spark plug in an engine cylinder for producing an ion current signal indicative of an ion current through the spark plug. The processing circuit includes a blanking circuit and an integrator. The blanking circuit is configured to suppress noise transients in the ion current signal. The integrator is configured to integrate the ion current signal that is absent of noise transients to produce an output signal. The output signal is indicative of knock, and in a preferred embodiment, is indicative of a knock intensity.
In a still further preferred embodiment, the processing circuit further includes a bandpass filter configured to filter the ion current signal from the ion current detection circuit. The bandpass is selected so as to pass a predetermined frequency range containing frequency components indicative of a knock condition. The bandpass filtered ion current signal is fed to a rectifier for rectification, and further to a threshold detector for producing a trigger signal when a noise transient exceeds a predetermined threshold. The blanking circuit is responsive to the trigger signal and is configured to suppress the noise transients in the filtered, ion current signal during a knock window.
Other objects, features and advantages of the present invention will become apparent to one skilled in the art from the following detailed description and accompanying drawings illustrating features of this invention by way of example, but not by way of limitation.